Two-wire transmitters are commonly used to monitor various conditions at remote locations. For example, to measure the liquid level in a tank at a remote processing plant from its central control room, a two-wire transmitter at the remote location is typically connected in series with a power supply and a load at a central location through two transmission wires. As the condition being monitored by the transmitter varies, the effective series resistance across the transmitter also varies so as to produce a corresponding change in the current drawn by the transmitter. An industry standard has developed in a large number of applications, wherein the current through the two-wire transmitter loop varies from 4-20 milliamperes (mA), wherein 4 mA is the minimum amount of current required to power the remote transmitter.
Volume, pressure, liquid level, and temperature are just some of the conditions which are typically monitored using two-wire transmitters. Temperature, however, is one of the conditions which often must be measured with precision. It is well known to utilize a resistance temperature device (RTD) for this purpose. The RTD is typically immersed in the medium, the temperature of which is to be measured, such that the resistance of the RTD will vary with the temperature changes of the medium. Utilizing either a table of resistance-temperature values or a polynomial equation to represent the relationship between the RTD's resistance and temperature, the actual temperature is then calculated from the measured resistance value of the RTD.
If the RTD is connected to the two-wire transmitter via two wire leads, then the RTD resistance measurement would necessarily include the resistance of the wire leads. For more accurate temperature measurements, a four-wire RTD system is often employed, i.e., two wires from each terminal of the RTD are connected to the two-wire transmitter. Two of the wires are used to pass current through the RTD, and the other two wires are used to sense the voltage developed across the RTD during the measurement. In this manner, the RTD's resistance is measured without passing current through the same wires that sense the voltages, i.e., without including the voltage drop of the lead wires. In still another version of an RTD system, a three-wire RTD is used, wherein such lead-length compensation is performed by measuring the voltage difference between only one voltage sensing lead and the current return lead. Numerous other RTD configurations are also possible, a few of which will be described below.
A problem often occurs whenever one of the wires to the RTD breaks or has an intermittent connection. Although a broken wire in the RTD's current path wires would immediately be apparent at the two-wire transmitter as an over-ranging, i.e., infinite, resistance measurement, a break in the voltage sensing wires may only slightly affect the resistance measurement by the amount of lead-length compensation being performed. In other words, depending upon the condition sensor configuration and the particular lead wire that is broken, a remote measurement system may appear to be functional yet be providing inaccurate readings for quite some time before the broken wire is discovered.
A need, therefore, exists for an improved remote measurement system which addresses the problem of detecting a broken wire in a three- or four-wire RTD temperature measuring unit.